A diaphragm of the kind described above is disclosed in, for example, the specifications of United Kingdom published patent application No. GB1 402 144A and U.S. Pat. No. 4,132,103. As is well known, the clutch diaphragm constitutes an axially acting resilient means exerting a force on, for example, a pressure plate of the clutch for gripping a friction disc or clutch friction plate. It is desirable that this force exerted by the diaphragm should remain constant with time. One of the major parameters which governs this is the temperature which the diaphragm may reach in service. For example, in the case of a friction clutch, the temperature of the diaphragm may reach high temperatures, in particular during uphill starting, when the friction liners may become very hot. This heat can be transmitted to the diaphragm, which may thus be subjected to temperatures of the order of 200.degree. C. It is important that the diaphragm should not slacken even during operation at these high temperatures.